This invention relates generally to liquid scintillation counting systems and, more particularly, to improvements in methods and apparatus for determining the degree of sample quenching in such counters.
Liquid scintillation techniques have been widely adopted to measure the count rate or activity of samples containing radionuclides. The radioactive sample, typically a beta emitter, is placed in direct contact with a liquid scintillation medium by dissolving or suspending the sample within the medium. The liquid scintillation medium comprises a solvent or solvents, typically toluene or dioxane, and solute or solutes present in a few percent by weight of the solution. The liquid scintillation solution consisting of the solvent(s), the solute(s), and the radioactive sample are placed within a sample vial for measuring the radioactive emissions within the liquid scintillator. It is theorized that most of the kinetic energy from the nuclear decay events of the radioactive sample is absorbed by the solvent and then transferred to the solute which emits photons as visible light flashes or scintillations. The amount of emitted light is proportional to the amount of energy absorbed from the decay events. The scintillations are detected by a photomultiplier tube or other light responsive device which converts the energy of each scintillation to a voltage pulse having a pulse height proportional to the energy of the detected scintillation.
To derive a pulse height spectrum of the test sample, the output electrical pulses from the photomultiplier are amplified and fed to a plurality of parallel counting channels of a multi-channel spectrum analyzer. Each channel typically includes a pulse height analyzer with discriminators which establish a channel counting "window" having upper and lower pulse height limits. Each counting channel therefore counts the total number of pulses produced having pulse heights within the window limits of the channel. By establishing plurality of counting channels having window settings which span a range of pulse heights and by counting the number of pulses falling within each channel, a pulse height spectrum is obtained for the particular radioactive sample. Since the output pulse heights from the photomultiplier are proportional to the energy of the corresponding scintaillations, the pulse height spectrum derived by the spectrum analyzer corresponds to the energy spectrum of the nuclear radiation emitted by the test sample.
It is well known that radioactive samples or materials present in a scintillation medium can adversely effect the process by which the scintillations are produced. For example, the emission of photons can be prevented or emitted photons can be absorbed. Further, some events can be reduced to a level which is below the minimum detection level of the photomultiplier. Such effects are commonly referred to as "quenching" and in each case result in a reduction in the number of photons detected by the photomultiplier. When quenching results in a reduction of the level of some events below the detection level of the photomultiplier, the measured count rate will be lower than that produced by the same amount of the radionuclide in an unquenched sample. This is commonly referred to as a decrease in "counting efficiency".
Quenching acts equally on all events produced by the same type of excitation particle, e.g. electron (beta), alpha, proton, etc. Thus if quenching is sufficient to reduce the measured response for one decay event by given percentage, it will reduce all given responses by the same percentage. The result is to shift the energy or pulse height spectrum to lower pulse height values, and this is commonly referred to as "pulse height shift".
A major effect has been directed to develop techniques for monitoring the level of quench and for correcting the measured pulse height response to compensate for the effect of quench. Several of the primary methods for measuring quench levels are the internal standard method, the sample channels ratio (SCR) method, the external standard counts (ESC) method, and the external standard channels ratio (ESCR) method.
Of the foregoing, the internal standard method is probably the oldest of the methods. In the internal standard method, a known amount of radionuclide of interest is added to a previously counted sample and the sample is recounted (with the radionuclide). The apparent increase in the counts provides an indication of the degree of quenching present.
While the measure of quench provided by the internal standards method is relatively accurate, the method is time consuming and tedious since it requires counting the sample twice. Moreover, the method presents a degree of hazard to an operator since it requires opening sample vials to add the internal standard radionuclide, typically by manually pipetting. Further, the radionuclide must be available in a form which can be added to the sample without altering the sample counting solution. In this regard, it is possible for the radionuclide to increase the quench level associated with the sample and, in the case of a contaminated pipette, to introduce variable amounts of quench which result in inordinately low counts for the radionuclide. Opening refrigerated samples to add the radionuclide can cause condensation of water, a strong quenching agent, within the sample containing vial. Beyond this, the internal standard method is destructive since, after combining the sample and the radionuclide, the sample can never be counted along again to recheck the original count.
In the sample channels ratio (SCR) method the sample spectrum is divided into two adjacent counting channels, and the ratio of counts in the two channels provides an indication of the degree of quenching. The ratio of counts in the two channels is calibrated with respect to radionuclide counting efficiency by measuring a series of standards of known radionuclide content and varying degrees of quench. The statistical accuracy of the sample channels ratio method is dependent upon the sample counting rate or total counts measured and, for this reason, the time required to obtain an accurate value of the sample channels ratio is often unacceptably long. As a result, the SCR method works well for high activity samples only.
The most common of the quench measuring methods are the external standard method (ESC) and the external standard channels ratio method (ESCR). Each employ an external gamma source to irradiate the liquid scintillator solution. A fraction of the gamma rays will interact with the scintillation solution to produce Compton scattering of electrons having a Compton scattered electron energy distribution or pulse height spectrum. The Compton scattered electrons are effected by quenching such that the resulting Compton scattered electron energy distribution or pulse height spectrum is shifted to lower pulse height levels.
In the external standard counts (ESC) method the sample is first counted alone. The external gamma source is then mechanically moved from a remote position to an operative position adjacent the sample vial, and counting is carried out for the gamms source in a specified counting window. A series of samples of varying quench level, but with known amounts of radionuclide, are measured to obtain a calibration curve of external standard counts or count rate as a function of sample counting efficiency. Thereafter, when an unknown sample is measured and the gamma source is moved into position to irradiate the sample, the degree of quenching in the unknown sample for the resulting sample count is derived from the calibration curve.
The external standard counts method has the advantages of being independent of sample activity and of being relatively rapid. However, the method is subject to significant errors because the number of Compton scattered electrons is affected by the volume of the sample solution, by the position of the gamma source relative to the sample, by the half-life of the gamma source, and by changes in electron density of the sample and its surroundings.
The external standard channels ratio (ESCR) method, described in U.S. Pat. No. 3,381,130, filed Aug. 16, 1965, and assigned to the assignee of the present invention, combines the use of an external gamma source and two counting windows. The Compton scattered electrons produce a distribution of pulses which are counted in the two windows and the ratio of the counts is used as a measure of the quench level. The Compton distribution is always the same at a given quench level, but will vary at different quench levels due to the pulse height shift caused by quench changes.
The external standard channels ratio method has the advantages of being rapid, independent of sample volume within certain limits, independent of the relative position of the gamma source and the sample, and independent of half-life of the gamma source. However, the method exhibits certain limitations since at some quench level, the ratio will become zero and further quench cannot be measured. Also as the ratio becomes small, the number of counts in one of the channels becomes small and the accuracy of the ratio becomes poorer.
For a sample having a given quench value, each of the above methods of quench determinations can provide different quench values depending upon, among other factors, how the user sets the counting windows for the counting operations. Thus, the same user measuring the same sample by the same quench determination method can obtain different values when, in fact, the sample has only one value of quench.
Another method of quench measuring which has been proposed in the literature and which also employs an external gamma source, involves measurement of the so-called "half-height" of the leading edge of the pulse height distribution of the Compton scattered electrons (this edge is commonly referred to as the Compton edge). The half-height of the Compton edge is established by measuring the peak height of the Compton pulse-height distribution to establish the count rate corresponding to the peak height. The peak count rate is then divided by two, and the location on the Compton edge which corresponds to this halved count rate is termed the "half height". The pulse height corresponding to this location is established and the relative shift of this pulse height for an unquenched and a quenched sample provides a measure of the degree of quench of the sample.
While the half-height method is theoretically attractive it presents certain problems of implementation. The major drawback of the half-height method is that it requires the peak height of the Compton pulse-height distribution to be measured. Unfortunately, it is difficult to measure the peak height with precision. First, it is necessary to count the sample for a long period of time in narrow windows around the peak in order to obtain sufficient counts to make a statistically accurate determination of the peak itself. Moreover, the difficulty in accurately defining the points is compounded as the degree of quench increases since the peak itself becomes more diffuse with increasing quench. Since the so-called half-height value is simply half of the peak value, it is evident that the half-height determination is no more accurate than the peak determination.
Improved liquid scintillation counters have been developed which monitor the level of quench using one of the above methods and which automatically make a correction to compensate for the quench. The effect of quench is to shift the pulse height spectrum to lower pulse height values thereby changing the relative position of the pulse height spectrum and the channel "windows" for counting the pulses. The automatic quench compensation methods, in effect, operate to re-establish the relative position of the pulse height spectrum and the channel windows. For example, British Patent Specification No. 1,226,834, corresponding to U.S. Pat. No. 4,029,401 filed July 3, 1967 and assigned to the assignee of the present invention, teaches several methods of automatic quench compensation by modifying selected system parameters in accordance with a measured quench value to restore relative positions of the counting channel "windows" and the pulse height spectrum. For example, the patent specification teaches that the gain of the photomultiplier tubes which detect the light scintillations may be adjusted to change the detected pulse heights and thus shift the pulse height spectrum. In addition, the patent teaches adjusting the window settings of the counting channels to shift the counting window to the correct region of the pulse height spectrum. Adjustment of the gain or channel window settings in the above manner automatically compensates for the level of quench.
While each of the previously described methods of measuring the level of the quench has proven satisfactory in some applications, they all suffer from one or more of the drawbacks enumerated above. As a result, automatic quench compensation systems incorporating any of these quench monitoring methods are inherently subject to the same drawbacks.